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Prof Miles said: "People are choosing to get into their cars, despite knowing they will end up stuck in traffic jams, because it is still a better option than the alternative - taking public transport into and out of the city."

"The only way to link up transport to the city with inner city transport is to go underground. It is the only option."

When asked how cost-effective tunnelling for transport is, the professor replied: “At first sight, tunneling is always more expensive.

"But when you look at a city scenario, particularly a very constrained city like Cambridge, putting transport on land is very expensive and time-consuming, and comes at a much higher cost.

The first conference of the Greater Cambridge Partnership, formerly the City Deal, at the Hauser Forum in Cambridge. Mayor James Palmer. Picture: Keith Jones

"It is always expensive to go underground, but the cost of the tunnels is proportionate to the size of the tunnel.

"So if you can deliberately keep the tunnels very small, and on the top you have a very congested urban space, then those things work to reverse the equation, and it can work out cheaper than going over the top.”

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Cambridgeshire and Peterborough Mayor James Palmer answered the same question, saying: "Clearly Cambridge is not a very big city. Because it isn't big, the amount of tunnels you need underneath it isn't very big either.

"But it is the controlled growth beyond Cambridge that the investors will want to know about - they won’t get the money back from the tunnels themselves, but from the opportunities they create.

"The London Underground was built with private investment and it has worked quite well - I don’t think we could imagine London without it.

"It is important to look for transport solutions for Cambridge that are going to last not just five or 10 years, but 50 to 100 years."

The first conference of the Greater Cambridge Partnership, formerly the City Deal, at the Hauser Forum in Cambridge. From Left: Mark Reeve, Mayor James Palmer, Lewis Herbert, Francis Burkitt, Ian Bates and Prof Phil Allmendinger. Picture: Keith Jones

Cllr Lewis Herbert pointed out a study on the feasibility was necessary.

He said: "Ultimately, we need to know how a study links to the deliverability of such a project.

"We will need to look at the projected usage... and who is going to fund it? Will it be private funded?